Development of new hybrid indirect composite resin for crown-TWiNY（Part4）Relationship between therefractive indices of resin components and the transparency of the resins /Kato T, Hoshikawa T, Nagai M, Yamamoto S(Yamamoto Precious Metal Co., Ltd.) : In this study, the influence on the transparency of the resin composite of the refractive index of aresin matrix (before and after polymerization), of the refractive index, and of the particle size of the inorganic filler were examined in order todevelop a new indirect composite resin, TWiNY which maintains high transparency throughout the entire polymerization process. Throughthis study, it was found that it is necessary to take into consideration the effect of light scattering due to the primary particle size of the filler,
as well as to adjust the refractive index of the filler, in order to produce a highly transparent resin composite.

[Abstract]
One of the major issues emerging with regard to resins
for use in crowns is the light transmittance(transparency) of the resin under photopolymerization,and its aesthetic characteristics. The transparency of aresin depends on the degree of difference between therespective refractive indexes of the matrix material(monomer and nano filler) of which the resin is mainlycomposed and that of the filler, along with the particle
diameter of the filler and its content by percentage.
Also, the refractive index of the matrix material
increases substantially when the monomers undergo
polymerization. In this investigation, with thedevelopment of a resin achieving high transparency as
the objective, the influence on transparency of therefractive index of the resin matrix (before and afterpolymerization), and the refractive index and particle
diameter of inorganic fillers, was studied in detailduring the resin photopolymerization process(before-during-after).

First, SiO 2 -Al 2 O 3 -ZrO 2 -type sol-gel particles weresynthesized through coprecipitation process; they thenunderwent aggregation, firing and coupling treatment
to create an aggregated inorganic filler (secondary
particle diameter: approx. 35 µm) 1) .
Resin (filler content by percent: 50 wt %) was preparedby use of a vacuum kneading machine, loaded in ametal mold (circumference: 15 mm; thickness: 0.8mm), both sides of the mold were covered with atransparent film, and test pieces for transmittancemeasurement were created. Transmittance before andafter photopolymerization was measured using aturbidimeter (NDH2000, produced by Nippon
Denshoku).

[Materials and Methods]
Table 1 shows the composition and refractive index of
the indirect composite resin used in this investigation.

[Results and Discussion]
Table 2 shows the specific surface area of the filler andthe transmittance achieved by the resin under these
conditions. Total light transmittance (natural dental
enamel: approx. 70% / 0.8 mm) showed practicalapplicability, but a marked decrease in paralleltransmittance accompanied the increase in primaryparticle diameter (Rayleigh scattering 2), 3) : deteriorationof aesthetic quality).

Table 1: Composition and Refractive Index of the Indirect
Composite Resin

Table 2: Specific Surface area of Filler and Optical
Characteristics of the Resin

StructuralcomponentResinmatrix

FillerContent[wt%]

UDMA

78.9

Colloidal silica
γ-MPTS

14.8
4.9

CamphorQuinone

0.4

DMAEMA
SiO2
Al2O3
ZrO2

1.0
87.1
0.8
12.1

Firing temperature [°C]

Refractive index [nd25]
BeforeAfterpolymerization polymerization

900

1000

11001150

1200

Specific surface area [m2/g]

85.1

48.7

22.5

12.8

4.8

Calculated particle diameter
[nm]

3060

120210560

Total
Light

74.5

77.6

73.3

68.4

60.8

parallel

57.5

25.5

10.7

5.7

4.4

Total
Light

81.5

75.5

78.0

76.0

68.6

parallel

63.1

41.0

33.9

16.9

7.8

Beforepolymerization

1.480

1.509

TransmittanceofResin[%]

1.494

The resin matrix material was of a photopolymerized
type strengthened with nano filler (SiO2; diameter: 15nm). Before polymerization, the refractive index was1.480; on the other hand, it became 1.509 after
polymerization. With the development of a resin
achieving consistent light transmittance before and after
polymerization as the objective, the refractive index of
the filler was set at (1.480 + 1.509) / 2 = 1.494.

Afterpolymerization

Next, we investigated the light transmittance of thematrix material undergoing photopolymerization,
process of increase in refractivity index from 1.480
before polymerization to 1.509 after polymerization.

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Each matrix with a refractive index of 1.48 – 1.52 wasc r e a t e d b y b le n d i n g T E G D M A , U D M A a n d
Bis-GMA, and the results achieved with the filler(Table 2, firing temperature: 1100 ˚C; 50 wt %) are
shown in Figure 1. Parallel transmittance values wereconsistent with expectations, but higher values weredisplayed when the disparity between the refractive
indexes of the filler and matrix were smaller, and thus
lower values were displayed when the disparity washigher. The transmittance of the resin was higher when
undergoing photopolymerization, implying that anadequate case depth was achieved.

Transmittance (%)

7060
5040

After polymerization

80●90Filler

Before polymerization

100Total Light

Diffusionparallel

30
20
100
1.460

1.470

1.480

1.490

1.500

1.510

1.520

1.530

Matrix refractive index [nd25]

Figure1: Characteristics of Optical Transmittance of filler(firing temperature: 1100˚C; content by percentage: 50 wt%)

From the above results, it is apparent that in order tocreate a resin with high transparency throughout to theresin photopolymerization process (before-during-after),
it is necessary not only to carefully calibrate the changes of
the refractive index of the matrix material that accompany
polymerization, along with calibrating the refractive index
of the filler, but it is also necessary to take into
consideration the influence that the primary particle
diameter exerts on permeabilitytransmittance values.
[Reference]
1) Hosokawa T, Miyazaki A, Kato T, Anraku T,
Yamamoto S: Japanese Patent Number: 4502673
2) Fujita M, al.: Journal of the Japanese Society for Dental
Materials and Devices, 5 (3), 427-434, 1986
3) Miyazaki A. et al.: Journal of Nippon Academy ofDental Technology，22 (1), 112-117, 2001

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